An urgent need exists to prevent the sexual transmission of HIV-1 to women. Worldwide, approximately 70% of all new cases are spread by sexual intercourse, with women more likely to be infected than men. The overall objective of this proposal is to define the role of the female reproductive tract (FRT) in preventing viral transmission and to understand how sex hormones modulate FRT HIV target cells to make women more susceptible to infection during the menstrual cycle. This proposal presents an innovative approach to test the hypothesis that during the window of vulnerability, sex hormones (estradiol, progesterone) and chemical contraceptives (oral preparations, subcutaneous implants, and vaginal rings) enhance FRT HIV-target cell receptor expression and infection/ transmission while decreasing interferon-stimulated gene (ISG) expression. Defined as the period during the menstrual cycle when women are most susceptible to HIV infection due to hormonal changes, the window of vulnerability is the time hypothesized by us when women are most likely to be infected by HIV. This is a novel approach for understanding the nature and mechanisms of HIV infection in the FRT, by testing the hypothesis that during the window, FRT HIV-target cell (CD4+T cells, macrophages, dendritic cells) receptor expression and infection/ transmission increases while interferon-stimulated gene (ISG) expression decreases, thus increasing the risk of HIV infection. This approach has 3 original Aims that will: 1) Define the changes in HIV receptor expression, activation and susceptibility to HIV infection of target cells (CD4+ T cells, macrophages and dendritic cells) in the vagina, cervix and uterus during the menstrual cycle; 2) Examine the direct and indirect effects of sex hormones/contraceptives on HIV receptor expression and HIV-susceptibility of target cells in the FRT; and 3) Determine to what extent sex hormones/contraceptives suppress intracellular ISG anti-HIV activity and increase HIV infection of target cells in the FRT. This study is unique in tht it integrates our understanding of the endocrine system and the immune system throughout the human FRT as it relates directly to the very cells most likely to be infected by HIV. Understanding how sex hormones/contraceptives specifically enhance HIV infection by increasing HIV receptors and decreasing intracellular viral protection will provide a basis of knowledge for developing therapeutic mechanisms to prevent transmission of HIV. It further provides a much needed foundation of information that should accelerate the development of multipurpose prevention technologies designed to provide women with safe, effective means of protecting themselves simultaneously from HIV, other sexually transmitted and reproductive tract infections, and/or unintended pregnancy.